A very significant addition to English halachic literature, Foundation of Purity is an extremely thorough sefer which includes clear explanations of the laws of family purity and the principles which underlie them. Based on the shiurim of the gaon, Rabbi Moshe Shaul Klein, shlita, this sefer is used extensively by rabbonim, poskim, Torah scholars and laymen as well. It is an invaluable resource for anyone who wants to become well-versed in the laws of niddah.